The cytogenetic aspects of ALL including the occurrence of sister chromatid exchanges (SCE) and their relation to the evolving cell populations as identified by chromosome markers is being studied. We have classified 57 newly diagnosed children with ALL into four karyotypic groups: normal chromosomes; hyperdiploid I with 49 to 64 chromosomes multiple, copies of the same chromosome and absence of a consistent clone; hyperdiploid II with a karyotype having 47 to 51 chromosomes and a recognizable clone; and pseudodiploids. Karyotypic findings have been considered in relation to those factors that have been found to be of prognostic significance by the Children's Cancer Study Group. SCE frequency in peripheral blood of these patients does not show significant differences among the groups.